* FTSEurofirst 300 down 0.6 pct, Euro STOXX 50 down 0.7 pct
* SABMiller, Nestle give weak updates
* Germany's DAX sinks as tension mounts in Ukraine
PARIS/LONDON, April 15 (Reuters) - Worries over escalating tension in Ukraine and weak updates by bellwethers including Nestle held back European stocks on Tuesday.
At 1428 GMT, the FTSEurofirst 300 index of top European shares was down 0.6 percent, at 1,311.35, resuming last week's slide during which the index lost about 3 percent, before slightly bouncing back on Monday.
The euro zone's blue-chip Euro STOXX 50 index was down 0.7 percent at 3,108.98 points, testing a key support level, its 50-day moving average, for the third straight session. Breaking below it would send a bearish technical signal.
"We're cautious in the short term, we're waiting to see how the geopolitical situation evolves. There are a lot of uncertainties about Ukraine, which could become a negative catalyst for stocks if things spin out of control," Barclays France director Franklin Pichard said.
Russia declared Ukraine on the brink of civil war on Tuesday as Kiev said an "anti-terrorist operation" against pro-Moscow separatists was under way, with troops and armoured personnel carriers seen near a flashpoint eastern town.
Germany's DAX index, seen as the most vulnerable to the crisis in Ukraine and sanctions against Russia, was down 1.3 percent, with Continental down 1.4 percent and Daimler down 2.6 percent.
"Things have not improved in Ukraine, and this is weighing on the markets," said Francois Savary, chief investment officer at Swiss bank Reyl.
Savary added that European stock markets now needed firm evidence of a recovery in earnings to move higher, after making little headway during the first quarter.
On Tuesday, several companies posted business updates that disappointed investors in some aspects.
Brewing company SAB Miller fell 2.5 percent after a sales update that some analysts said failed to meet forecasts.
French cosmetics group L'Oreal and Nestle both undershot market expectations with first-quarter sales, although they forecast a return to top-line growth in coming quarters.
Europe bourses in 2014: link.reuters.com/pap87v
Asset performance in 2014: link.reuters.com/gap87v
Today's European research round-up (Editing by Alison Williams)